1. Field of the Invention
The present invention relates to a photocathode, and more particularly, to a photocathode structure having an ultra-thin protective layer coated on the surface of a photoelectric face plate, in order to prevent performance of the photoelectric face plate from being deteriorated sharply due to oxidation in the case that the photoelectric face plate used for generating photoelectrons by a photoelectric effect is exposed to the atmosphere, and a material used for realizing the photocathode structure.
2. Description of the Related Art
A photoelectric effect is a phenomenon that electrons are externally emitted from the surface of metal when photons having an energy greater than a limited oscillation number are incident to the electrons on the metal surface. A photocathode has been being made of a material capable of emitting photoelectrons well by using the photoelectric effect. Thus, devices or apparatuses for transforming the photons incident to the photocathode into photoelectrons have been under development. The devices using the photocathode are representative of a photomultiplier tube, a photo analyzer, a gamma ray camera, a positron CT (Computer Tomography) and a flat panel display using a MCP (Microchannel plate).
In the case of devices or apparatuses using the photoelectric effect, the features of the material used for the photoelectric face plate become the most important factor which influences upon the whole characteristics of the device or apparatus. The features of the materials used for the photoelectric face plate have been enhanced greatly through the research and development up to now.
The materials used for a currently commercialized photocathode are classified into a material having alkali metal of a low work function as a main component and a material having a gallium arsenide (GaAs) semiconductor as a main component. There are Csxe2x80x94I, Csxe2x80x94Te, and Sbxe2x80x94Cs each having one kind of alkali metal, Sbxe2x80x94Rbxe2x80x94Cs, Sbxe2x80x94Kxe2x80x94Cs, and Sbxe2x80x94Naxe2x80x94K each having two kinds of alkali metal, and Sbxe2x80x94Naxe2x80x94Kxe2x80x94Cs each having three kinds of alkali metal, as the materials of the photoelectric face plate having alkali metal as a main component. There are GaAs(Cs) and InGaAs(Cs) as the materials of the photoelectric face plate having GaAs as a main component. Besides, there is Agxe2x80x94Oxe2x80x94Cs or the like. In the case that alkali metal is used for the photoelectric face plate, the photoelectric material is easily oxidized due to high reactivity of the alkali metal, to thereby lower a quantum efficiency. Thus, for this reason, the processes of fabricating or assembling the devices or apparatuses after having been fabricated the photoelectric face plate should proceed at the state where the photoelectric face plate is isolated from the atmosphere, that is, under the vacuum circumstances.
In the case that p-type doped gallium arsenide (p-GaAs) is used as a material for a photoelectric face plate, cesium (Cs) or oxygen (O) is adsorbed in the photoelectric face plate, to thereby make a photoelectric material having twice the quantum efficiency of alkali metal. However, the deposition equipment for depositing GaAs is expensive and the deposition process is complicated. Also, since GaAs is deposited using toxic gas, paying a careful attention is required during processing. Also, in order to use the deposited photoelectric material as a photocathode, impurities are removed from the surface of the photoelectric material and then the impurities removed photoelectric material should be vacuum-sealed. If the impurities removal is imperfect or the vacuum sealing is not perfectly done in the sealing process, the photoelectric material cannot be used as the photocathode. That is, the complicated process causes an actual production efficiency to decrease.
U.S. Pat. No. 5,977,705 discloses a technique of solving the problems occurring when the alkali metal or GaAs is used as the material for the photoelectric face plate, in which diamond-like carbon or diamond or the combination of both is used as the material for the photoelectric face plate. The technique of U.S. Pat. No. 5,977,705 uses cheaper deposition equipment than that used when GaAs is used as the material for the photoelectric face plate, and does not use any toxic gas. Thus, the process can be simplified and appropriate for mass-production. However, the diamond-like carbon or diamond has a negative electron affinity, and thus can be used for the photoelectric material. Nevertheless, since the quantum efficiency is smaller than the existing photoelectric material, a material such as Cs, O or H should be added, in order to exhibit an appropriate performance as a photoelectric face plate. In the case that the Cs, O or H material is added in the GaAs photoelectric material, the material having a high reactivity causes the subsequent processes to be performed in a vacuum.
FIG. 1 shows the structure of a flat panel display using a conventional photocathode. Referring to FIG. 1, the flat panel display using a conventional photocathode will be described below to review some problems in the conventional photocathode.
In the FIG. 1 flat panel display, light 4 is emitted from light emitting devices 11 which are arranged in one surface of a substrate 10 which is driven by a transmitted electrical image signal. The light 4 emitted from a light emitting devices array 12 formed of light emitting devices, for example, hydrogenated amorphous silicon-carbide (a-SiC:H) is not so sufficiently bright as to be used for image display. Amplification of the light is needed. In the case of a flat panel display using a microchannel plate (MCP) 14 to amplify the light 4, a photocathode 13 is used for transforming of the light 4 emitted from a light emitting devices array 12 into photoelectrons. The light emitted from the light emitting devices array 12 is incident to the photocathode 13 which is very near from the light emitting devices array 12. Then, photons are transformed into photoelectrons, the transformed photoelectrons are multiplied via the MCP 14. The photoelectrons multiplied by the MCP 14 are accelerated and collided with fluorescent materials which emit light of red (R), green (G) and blue (B) which are arrayed and coated regularly on one surface of a transparent substrate 16, to thereby excite the fluorescent materials respectively to emit corresponding color light.
With the above-described flat panel display, it is possible to fabricate a thin and large-area flat panel display in theory. Also, if light emitting devices for emitting infrared region light as well as light emitting devices for emitting visible light are used, a perfect color display is accomplished. However, in the case that alkali metal such as Sb, Na, Cs or the like having an emission sensitivity distribution with respect to the light of a wavelength between 400 nm and 900 nm is used as a material for a photoelectric face plate, the alkali metal having a high degree of reactivity is easily oxidized on the surface at the time of exposure to the atmosphere, to thereby reduce a quantum efficiency and lower performance of the whole flat panel display.
Also, the flat panel display using the above-described conventional photocathode raises the problem in the photocathode deposition process and the process subsequent to the photoelectric material deposition process. In the photoelectric material deposition process, a photoelectric material should be deposited on a transparent substrate in order to enable the photons emitted from a light emitting device to be incident to a photocathode. Also, a material having a high conductivity should be used as a substrate in order to apply a uniform voltage to the large-area photoelectric face plate. That is, in this case, a transparent substrate on which a transparent conductive layer is coated should be used as a substrate. The conventional transparent conductive layers are made of ZnO, In2O3, SnO2 or the like. However, since such transparent conductive layers are oxides, they react with the deposited photoelectric material, to thereby raise the problem that the photoelectric material is oxidized. Since the photoelectric material has a high reactivity, the process subsequent to the photoelectric material deposition process should proceed at a vacuum state which is isolated from the atmosphere. In order to perform a chain of the subsequent processes in the vacuum, a complicated control unit is required and is very difficult to achieve the control technique. Thus, the subsequent processes in the vacuum become factors which raises a manufacturing cost of a device or apparatus. As described above, the degradation problem of the photoelectric face plate due to a high reactivity of the existing photoelectric materials should be solved.
To solve the above problems, it is an object of the present invention to provide a photocathode having an ultra-thin protective layer in which the features of EL photoelectric face plate can be maintained although the photocathode is exposed to the atmosphere, and a transparent conductive layer does not react with the photocathode although an existing transparent conductive layer is used as a cathode material.
To accomplish the above object of the present invention, there is provided a photocathode having an ultra-thin protective layer comprising: a transparent substrate; a photoelectric face plate which is deposited on the transparent substrate and transforms light incident through the transparent substrate into electrons and emits the transformed electrons; and a first photoelectric face plate protective layer which covers the surface of the photoelectric face plate and isolates the photoelectric face plate from the atmosphere.
Here, the electrons emitted from the photoelectric face plate pass through the first photoelectric face plate by the tunneling effect.
Also, in order to apply a uniform voltage to the large-area photoelectric face plate, a transparent substrate (for example, a transparent conductive plate) on which a material having a transparent and high electrical conductivity (for example, a transparent conductive material) is coated should be used as a substrate. In the case that a photoelectric material is deposited on the substrate, a second photoelectric face plate is deposited and interposed between the photoelectric face plate and the transparent conductive layer in order to prevent the problem that the transparent conductive material reacts with the photoelectric material and thus the photoelectric face plate is oxidized.